Flight at speeds well above the local velocity of sound. In the span of a few years speeds of aircraft, missiles, and spacecraft increased tremendously, from far less than the speed of sound (subsonic) to speeds required for orbiting or escape from the Earth. By convention, hypersonic flight starts at about Mach 5 (five times the speed of sound) and extends upward in speed indefinitely.

A more precise definition, attributed to T. von Kármán, states that hypersonic flight starts when the cross-flow Mach number (or Mach-number component perpendicular to the longitudinal axis) becomes 1 or greater. In any case, hypersonic flight refers to atmospheric flight at hypersonic speed when the characteristics of the flow field about the body are sensibly unchanged as the speed of the body increases further.